Light energy is characterized by a dual nature both from a quantum point of view as photons and from a wave point of view as randomly polarized electromagnetic radiation with wavelengths from 400 nm to 700 nm. If the ultraviolet and infrared portion of the spectrum is included, the range of wavelengths is extended at both extremes. Presently all practical solar cell energy collection schemes use the photon nature of light in the explanation of operation. The conversion of solar energy to electrical energy using the photovoltaic effect depends on the interaction of photons with energy equal to or greater than the band-gap of the material. With continued research the maximum amount of energy captured using this mechanism is estimated to be around 30%. This is equivalent to an efficiency of 30%.
Another mechanism known in the art for converting solar energy to electrical energy is the rectenna. Rectenna technology relies on the electromagnetic nature of radiation and is not limited by the band-gap of the rectifying material. Advantages exist in the rectenna field because the rectenna approach is not fundamentally band-gap limited. Also, at microwave frequencies, 2.45 GHz, this concept has been demonstrated to be approximately 90% efficient.
Prior art exists in the field of rectenna technology. The idea of direct electromagnetic conversion of solar energy has been explored as early as 1973. The concept envisioned small cone type antennas with rectifiers mounted behind the cones and the cones arranged in an array as illustrated in FIG. 1. The work preceded the revolution in micro and nano scale manufacturing and was never realized. Other more recent researchers have replaced the cones with planar antenna elements such as half-wave dipoles, bow tie, and spiral antennas.
The previous efforts in researching rectennas for solar energy conversion have concentrated on either building the rectifiers on a supporting substrate and then building the antennas on top of the rectifying layer, or fabricating the antenna structure and the rectifier on a planar surface. The fabrication processes known in the art for rectenna elements rely on hybrid systems that incorporate different technologies for each component of the rectenna to optimize the system performance. While the incorporation of hybrid technologies often allows for a shorter development time, compromises must be made to ensure that each component can be fabricated without damaging components already existing on the substrate.
By contrast, monolithic device fabrication allows for all components of a system to be constructed on a single substrate utilizing one technology. Monolithic devices are typically more compact than hybrid devices, and more reliable due to the reduction in interconnections required. Additionally, once the fabrication process for a monolithic device has been developed, they can be manufactured more efficiently since less assembly is required. Monolithic technology minimizes relatively long, high-resistivity connections and high loss metal interconnects.
Accordingly, what is needed in the art is a monolithic rectifying antenna providing high efficiency solar energy conversion with improved reliability and manufacturability.
However, in view of the prior art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the pertinent art how the identified need could be fulfilled.